The C-glycoside derivative represented by the formula (1) and its salt [hereinafter, they are referred to as “compound (1)” or “compound of formula (1)” in some cases] is known to be useful for treatment and prevention of diabetes such as insulin-dependent diabetes (type 1 diabetes), non-insulin-dependent diabetes (type 2 diabetes) and the like and various diabetes-related diseases including insulin-resistant diseases and obesity (Patent Literature 1).

The method for producing the C-glycoside derivative represented by the formula (1), described in the Patent Literature 1 is understood to be represented by the below-shown reaction formula (I), by referring to the Examples and Reference Examples, described in the Patent Literature 1. Roughly explaining, it is a method which comprises reacting [1-benzothien-2-yl(5-bromo-2-fluorophenyl)methoxy]tert-butyl)dimethylsilane (synthesized in accordance with Reference Example 37 of the Literature) in a manner shown in Example 65 of the Literature, to obtain (1S)-1,5-anhydro-1-[3-(1-benzothien-2-ylmethyl)-4-fluorophenyl]-2,3,4,6-tetra-O-benzyl-D-glucitol and then reacting the obtained compound in accordance with Example 100 of the Literature to synthesize intended (1S)-1,5-anhydro-1-C-[3-(1-benzothiophene-2-ylmethyl)-4-fluorophenyl]-D-glucitol.

However, the method for producing the C-glycoside derivative of the formula (1), disclosed in the Patent Literature 1 is not industrially satisfactory in yield and cost, as is seen in later-shown Reference Example 1 of the present Description.
For example, as described later, the method includes a step of low product yield (for example, a step of about 50% or lower yield) and the overall yield of the C-glycoside derivative (final product) represented by the formula (1) from the compound (8) (starting raw material) is below 7%; therefore, the method has problems in yield and cost from the standpoint of medicine production and has not been satisfactory industrially. In addition, the method includes an operation of purification by column chromatography which uses chloroform as part of purification solvents; use of such a solvent poses a problem in environmental protection and there are various restrictions in industrial application of such an operation; thus, the method has problems in providing an effective medicine.
Also, an improved method of conducting an addition reaction with trimethylsilyl carbohydrate instead of benzyl carbohydrate and then conducting deprotection for acetylation, is known for a compound which has a structure different from that of the compound of the formula (1) but has a structure common to that of the compound of the formula (1) (Patent Literature 2). It is described in the Patent Literature 2 that the improved method enhances the overall yield to 6.2% from 1.4%. Even in the improved method, however, the yield is low at 6.2% which is far from satisfaction in industrial production.    Patent Literature 1: WO 2004/080990 Pamphlet    Patent Literature 2: WO 2006/006496 Pamphlet